Wireless communication networks are critically important for maintaining coordination and intercommunication between elements of mobile combat assets. Frequently, it is necessary during a mission to maintain communications over large distances, which can require that transmissions be relayed to their final destinations. This can be enabled by assigning nodes within the network to function as active relays. Of course, it is important that secure, tactical networks remain resilient in the presence of jamming threats.
An example of a wireless network with good anti-jamming (“AJ”) features for secure military communication between mobile assets is the Link 16 networking protocol. Link 16 is a widespread tactical wireless networking system that is used by frontline land, air, and naval systems in the United States, NATO, and allied nations to allow multiple users to share situational awareness data.
Information is transmitted on a Link 16 network in TDMA timeslots that repeat every frame, or “epoch.” The total number of timeslots included in a Link 16 network can be divided into subsets that represent virtual subnetworks, also referred to as “subnets.” Each subnet is distinguished according to the subset of the Link 16 time slots that belongs to the subnet, as well as by the participants that share the subset of time slots. Link 16 subnets are also differentiated by their frequency-hopping patterns. Multiple subnets in a network can be “stacked” or “multinetted” by allowing time slots to be used redundantly, with the data transmitted in each net on different frequencies (FDMA) and possibly also with different coding (CDMA).
A typical link 16 network is shown in FIG. 1. The blocks 10 in the ring 12 are time slots. Each participant 14 is provided transmit and receive time slot assignments by a network planner (not shown) prior to start of a mission. The column 16 to the right of the ring 12 illustrates the ability for Link 16 to operate on multiple nets (shown as stacked rings in the column 16). Each of the rings in the column 16 can be replaced, allowing users to form sub-networks or sub-nets allowing them to exchange data using different CDMA and FDMA codes to expand the capability of the network.
Each Link 16 participant terminal is initialized with a unique identifier, known as the Source Track Number (STN), along with time slot assignments that indicate which time slots are to be used for transmitting and receiving. Time slots can also be assigned for relaying of information by designated relay nodes in the network.
Given that the transmit power for Link 16 radios is typically 200 Watts, and the communications range for Link 16 is approximately 300 nautical miles, relays are almost always required for large operational areas. Currently, the relay assignments are established in advance, during network design, and time slots are assigned for the relay function as part of the network design. Currently, the Link 16 protocol defines three relay modes, which are “unconditional,” “conditional,” and “suspended.” Nodes that are assigned to the unconditional relay mode always relays messages received in the assigned relay time slots, regardless of location. Nodes that are assigned to the suspended mode never serve as relays.
Nodes that are assigned to the conditional relay mode are instructed to relay messages as needed, and are selected at any given moment according to which of the conditional relay nodes has the greatest geographical coverage at that time, as defined by its height and range, which are reported in Precise Participant Location and Identification (PPLI) messages exchanged between the nodes. Details of the construction of the PPLI messages can be found in MIL-STD-6016. Typically, relay nodes are selected based on bandwidth availability, based purely on node location, or on some other, somewhat arbitrary basis. In the case of aircraft, the selected conditional relay node is often the one with the highest altitude.
Despite its inherent anti-jamming features, the effective communication range of a Link-16 network can nevertheless be significantly reduced by an interfering adversary transmitting a focused, localized, high power jamming signal toward the Link-16 nodes.
What is needed, therefore, is a method for maximizing the effective communication range of a wireless, tactical communication network such as a Link-16 network when nodes in the network are subjected to a high power, localized interference signal.